1. Field of the Invention
The present invention relates to a cylindrical elongated vertical furnace for treatment of material at high temperature, preferably above 1000.degree. C in a gaseous atmosphere under high pressure, preferably above 500 bar.
2. The Prior Art
Pressure furnaces involve many constructional problems compared with furnaces operating at atmospheric pressure or lower pressure. The furnace space must be enclosed in a pressure chamber capable of confining gas under high pressure. This means that the cost of the furnace per unit of volume will be high. The cost increases rapidly with increased pressure and increased pressure chamber diameter. This means that it is necessary to economize the space in the pressure chamber. The heating of the pressure chamber walls must be limited in order to be able to obtain and maintain the desired treatment temperature with a reasonable supply of energy. The insulation and the heater inside the pressure chamber between the furnace space and the pressure chamber walls must be designed with the smallest radial extension in order to obtain a maximum furnace space. Designing the furnaces so that a furnace space with a large diameter is obtained in a pressure chamber with a small diameter involves many difficult problems. In the U.S. Pat. Nos. 3,598,378, 3,628,779 and 3,790,339 pressure furnaces of various designs are described in greater detail. In all these furnaces there is an insulating sheath between the heater and the pressure chamber wall, said sheath containing a number of metal tubes and an insulation applied between these tubes. Insulating sheaths in which at least the innermost or the inner metal tubes are freely suspended in a supporting member have been found to possess particularly good insulating properties. However, also these have proved to have a shorter life than desirable. The lower parts of the inner tube or tubes have buckled during the operation of the furnace. In all essentials this buckling has restricted itself to the parts of the tubes which are strongly cooled by the in-flowing pressure medium when the furnace is filled. The buckling has shown a tendency to increase continuously, which means that the inner tube sooner or later will come into contact with the heater or heating elements supported by the heater, or the leads of said heating elements. This will result in short-circuits or other damage to the heater so that it must be replaced. The buckling makes it difficult or impossible to withdraw the heater without its being totally destroyed. The buckling also means that the inner tube of the insulating casing has to be exchanged.